WASHINGTON — Kentucky will be the first state to require many of its Medicaid recipients to work or face losing their benefits after the Trump administration approved its plan on Friday.

Advocates for the poor threatened lawsuits, while Gov. Matt Bevin, a Republican, celebrated the approval as “the most transformational entitlement reform that has been seen in a quarter of a century.”

The plan calls for most Medicaid recipients who are not disabled and aged 19 to 64 to work at least 20 hours a week, beginning in July. In addition to paid jobs, they could meet the requirement through volunteer work, job training, searching for a job, taking classes or caring for someone elderly or disabled.

Pregnant women, full-time students, primary caretakers of dependents and the chronically homeless will be exempt from the work requirement, as will people deemed medically frail. But the Bevin administration still expects about 350,000 people to be subject to the requirement, which will be phased in around the state starting in July. About half of them already meet it, according to the administration.

“We are ready to show America how this can and will be done,” Mr. Bevin said at a news conference in Frankfort. “It will soon become the standard and the norm in the United States of America, and America will be better for it.”

Roughly 500,000 adults have joined Kentucky’s Medicaid rolls since the state expanded the program under the Affordable Care Act in 2014. Mr. Bevin has consistently attacked the expansion as a waste of money, questioning why “able-bodied” adults should be given free government health care that used to be largely limited to children, the elderly and the disabled.

He filed for federal permission to impose work requirements in 2016 — notably, instead of seeking to end the state’s Medicaid expansion altogether. And since then, more than a dozen other states have also sought work requirements or said they plan to. Several sought Medicaid work requirements during the Obama administration but were rebuffed.

The approval came just a day after the Trump administration gave states the O.K. to impose work or other “community engagement” requirements as a condition of getting Medicaid. According to the nonpartisan Kaiser Family Foundation, 60 percent of working-age Medicaid recipients who aren’t disabled already have full- or part-time jobs.

Under its plan, Kentucky will also require many adults who aren’t elderly or disabled to pay premiums of $1 to $15 a month, depending on their income. And it will disenroll people from Medicaid for up to six months if they fail to report changes in income or work status. Those who qualified for Medicaid under the Obamacare expansion will also have to “earn” dental and vision benefits, which they have been able to access freely until now, through activities like taking a financial literacy course or getting a GED.

The Bevin administration has estimated that the plan will result in 100,000 fewer Medicaid recipients after five years and save $2.4 billion, mostly in federal Medicaid funds. But Mr. Bevin couched the policy change as a moral rather than a fiscal decision, saying he did not care about the savings and saw it as an opportunity for Kentucky’s poor “not to be put into a dead-end entitlement trap but rather to be given a path forward and upward so they can do for themselves.”

Advocates for Medicaid beneficiaries said they disagreed with the Trump administration’s assertion, in approving Kentucky’s plan, that work requirements were consistent with the goals of Medicaid because work could improve people’s health.

“Considering that it will seriously harm over 100,000 Kentuckians, in violation of numerous provisions of Medicaid law, we are very seriously considering taking legal action — and as we analyze the meager legal rationale in the approval itself, it seems inevitable,” said Leonardo Cuello, director of health policy at the National Health Law Program, an advocacy group for the poor.

Emily Beauregard, the executive director of Kentucky Voices for Health, an advocacy group, said the state had provided little information about how it would make sure people were complying with work requirements, how exemptions would be determined and other details.

“We’re anticipating Kentuckians by and large are going to be extremely confused and worried about what they’re going to face and whether or not they’ll continue to have coverage,” Ms. Beauregard said. “They’ll be looking to advocates and enrollment assisters and their providers for answers, and at this point we don’t have any.”

She added, “The idea that we are encouraging work and independence, then taking away the health care that makes people more employable and better able to function — none of this adds up to something that’s going to be good for Kentuckians or our economy.”

But Hal Heiner, Kentucky’s Education and Workforce Development secretary, said during Mr. Bevin’s news conference that there was “an abundance of jobs” available to Medicaid recipients, as well as resources to prepare them.

“We have the jobs, we have the tuition resources, we have the job coaches in our career centers all across the state,” he said, “and now we’ll be able to connect the dots.”

Other state officials said the state was building an IT system to track people’s compliance with the work and premium requirements and participation in activities, like taking the financial literacy course, that would earn them points toward dental and vision care. They did not, however, provide a cost estimate for building and maintaining the administrative infrastructure necessary to monitor compliance with the new requirements.

Kentucky’s uninsured population has dropped more than almost any other state’s under the Affordable Care Act, and several studies have found significantly increased access to primary care, preventive screenings and care for chronic conditions there since the Medicaid expansion. But the state’s population remains unhealthy overall, which Mr. Bevin pointed to as proof that the Medicaid expansion was not working.

“The idea that we should keep doing what we’re doing is an insult to the people of Kentucky,” he said.

Sheila Schuster, a longtime health care advocate in the state, said she saw it differently.

“The administration has their chicken-and-egg story completely wrong — they say people need to work to get healthy,” she said. “We all know that health is the foundation from which people go to school, go to work and keep their employment. So I’m afraid the administration is not only going backward, but doing it for completely the wrong reasons.”

Such opposing views were evident in comments people posted on Mr. Bevin’s Facebook page during his news conference, which was livestreamed there. “ABOUT TIME to get others to pull their weight!” one viewer in favor of the new requirements wrote.

“I feel this is wrong,” another said. “Wouldn’t they not be in Medicaid if they could get a job?”